(1) Field of the Invention
The present invention pertains generally to encoding of pictorial data and, more particularly, to encoding of two level (black and white) facsimile pictures using run length coding.
(2) Description of the Prior Art
In U.S. Pat. No. 4,060,834 issued to F. W. Mounts and A. N. Netravali on Nov. 29, 1977, a system for increasing the run length of digital signals is described wherein, in the context of facsimile signals, a prediction error signal is reordered into two groups in accordance with a reference signal which indicates the degree of confidence in the predicted error values. This system provides the advantage that longer runs are formed, since prediction errors are less likely to occur in the good group. This in turn permits more efficient coding. An extension of the aforesaid Mounts et al system is described in an application Ser. No. 926,330 filed July 20, 1978 by the present applicants and assigned to the same assignee. In that system, certain runs in the reordered data are dropped, while the lengths of certain other runs are combined or bridged. Also, different run length code dictionaries are used for the runs in each group, without the need to encode any indicia of which code dictionary was used.
While the aforementioned techniques are effective in improving the efficiency with which facsimile signals may be encoded, it has been found that with a slight modification of the original picture data, by elimination of all horizontal runs of "black" or "white" picture elements which are of length one, a still further and significant decrease in entropy can be achieved. This improvement, furthermore, does not excessively degrade the picture quality, where the original picture is of high resolution (e.g., 200 dots/inch).